Ceramic articles, and a process for the production thereof

ABSTRACT

A ceramic sealing component for a machine which is in frictional contact and has at least one non-reflective sliding and sealing face of a mean roughness (Ra) greater than 0.1 and a minimum of 0.8 μm is described. A component of this type can be produced by subjecting a green ceramic article to shaping, sintering, processing by face-grinding or round-grounding, optionally washing and then abrasive-polishing in the presence of an aqueous phase until the mean roughness desired for the machine component is reached.

The invention relates to a process for the production of ceramiccomponents which have at least one planar or curved sliding and sealingface.

Ceramic objects having a planar or curved sliding and sealing face areusually produced by shaping a green ceramic article, sintering, grindingand subsequent lapping or polishing the ground face. Sliding rings forgaskets or sealing and regulating washers for sanitary fittings areexamples of objects produced in this way. During polishing, the moldingsare treated for a relatively long period with a paste or emulsion of apolishing agent, for example diamond grit, and at the same time pressedagainst rotating surfaces, for example grooved planar steel disks, inspecial-purpose automatic polishers. Rough areas are leveled in thisway. Machines for lapping and polishing require high investment; theiroperation is labor-intensive and therefore associated with high laborcosts per unit.

For aluminum oxide sliding rings, a maximum permissible unevenness of0.6 μm is required for adequate sealing. The mean roughness (Ra) shouldnot exceed 0.15 μm (VDI Reports (sic) No. 194, 1973, p. 124). Suchroughnesses can only be achieved by polishing or lapping.

European Offenlegungsschrift 043,456 discloses valve washers for mixingvalves, in particular sanitary mixing faucets, which contain zirconiumoxide or hafnium oxide and have a mean roughness of less than 0.3 μm.Treatment by lapping or polishing is necessary for the production ofthese washers.

In the visual control of customary ceramic articles having a sliding andsealing face, those skilled in the art require that this face reflects.

The object was therefore to specify a process for the production ofceramic moldings which have planar or curved sliding and sealingsurfaces, and with the aid of which it is possible to produce in asimple and inexpensive manner, surfaces of identical sliding and sealingbehavior.

The present process achieves this object. This is a process for theproduction of ceramic sealing components for a machine, the componentsbeing in frictional contact and having at least one planar or curvedsliding and sealing face, by shaping a green ceramic article, sintering,grinding and subsequently smoothing the sliding and sealing face formedand, if necessary, subsequently washing. This process comprisesabrasive-polishing the articles in the presence of an aqueous phase,without using scouring elements, until a mean roughness of greater than0.1 and not more than 0.8 μm is reached, the articles used being shapedin such a way that they can develop their abrasive-polishing effect onthe sliding and sealing face being formed.

In abrasive polishing, small parts, which are usually present in largenumbers, are agitated in a container, together with an aqueous phase,for a relatively long period. This process is used for deburring orpolishing metallic parts in electroplating.

This process is described in greater detail in the reference book"Trommeln" by A. Linek, Deutscher Fachzeit-schriften- und FachbuchverlagGmbH, Stuttgart, 1953. It is stated that the metallic parts are agitatedin the presence of added abrasive elements, for example sand or "Trowal"(synthetically prepared aluminum oxide). The treatment of ceramicsurfaces is not mentioned in this literature citation.

The process according to the invention can be applied, in particular, tothe smoothing of the surfaces of articles made from silicon carbide,steatite and aluminum oxide.

After grinding, the mean roughness Ra is 0.8 to more than 1 μm,depending on the abrasive disk used. It is possible, but in most casesnot necessary, to wash and thus to free of grinding dust, the groundceramic articles before abrasive polishing. Washing or degreasing isappropriate, in particular, if the ceramic articles are contaminated bygrinding oil. In this refinement of the process according to theinvention, finely divided abrasive substances, such as sand, are notpresent. In the case of planar sliding faces, the grinding takes placeby face grinding, and in the case of curved faces by circular grinding.

Abrasive polishing takes place in a scouring apparatus. Scouring drums,for example, in which the ceramic articles undergo tumbling movementssimilar to in a washing machine can be employed. This is favorable if,in addition to the smoothing of the sliding and sealing face, roundingof the edges is desired. Furthermore, scouring vibrators, in which thearticles undergo short oscillating movements of high frequency can beemployed. In these vibrators, relatively large parts, in particular, canbe treated without danger of damage. The scouring apparatus is usuallyfilled to 10 to 90% by volume, preferably 30 to 60% by volume, with theceramic articles which are to be polished by abrasion. Some of thevolume of the apparatus, for example 0 to 30%, in particular 0 to 10% byvolume, can remain empty. The remainder is filled with aqueous phase.The amount of water should be at least large enough so that all ceramicparts are covered with water, even during the scouring process. Thecorrect degree of filling can be determined by simple experiments and isusually in the range 15- 45% by volume. The scouring time is severalhours, for example 5-40 hours, and depends somewhat on the adjustableagitation intensity. Scouring sieves are also suitable, even if thenecessary time is longer than in the case of vibrators. Measured fromthe mean roughness, the smoothing process proceeds rapidly at thebeginning and more slowly later, so that there is hardly any danger ofobtaining abrasive-polished articles having an undesirably lowroughness.

If the amount of ceramic articles in the scouring machine is low, toofew abrasion partners are present; in this case, the smoothing effect isnot ideal and the possibility of fracture exists due to the vigoroustumbling. If the amount of ceramic articles is too great, the relativemotion between the parts is insignificant, which leads to excessivelylong processing times.

The process according to the invention is suitable, in particular, forthe production of sealing and regulating washers (for sanitaryapplication), sliding rings, plungers, shafts, protective sleeves forshafts, bearing rings and balls for ball valves.

It has been shown that a particularly smooth surface is obtained if asurfactant, for example, soft soap, is added to the treatment liquid inamounts of 0.05-20 g/l, preferably 0.1 to 10 g/l. In this way, theformation of scratches is suppressed and the speed of the smoothingprocess is increased.

There are ceramic moldings in which the sliding face or sealing facefaces inwards or, at least, is in such an unfavorable position that theelements are to that extent unable to develop a mutualabrasive-polishing action. This applies, for example, to certainprotective sleeves for shafts, bearing rings and to combined sliding andbearing rings. In these, the sliding and sealing faces are in theinterior. In this case, it is necessary to work in the presence of smallscouring elements which are free of sharp edges and which are able toreach the faces to be scoured

The invention furthermore relates to a process for the production of aceramic sealing component for a machine, the component being infrictional contact and having at least one planar or curved sliding andsealing face, by shaping a green ceramic article, sintering, grindingand subsequently smoothing the sliding and sealing face formed, whichprocess comprises abrasive-polishing the article in the presence of anaqueous phase with the addition of scouring elements, but in the absenceof finely divided abrasively acting substances until a mean roughness ofgreater than 0.1 and not more than 0.8 μm is reached.

Examples of scouring elements which can be used are spheres or rods ofceramic material, for example spheres whose diameter, and rods whoselength, is 3-10 mm. Scouring elements having a density of 2 to 4g/ml,preferably scouring elements of the same composition as the ceramicarticle to be smoothed, are preferred. The scouring elements cancomprise Al₂ O₃, SiC or boron carbide. If working in the presence ofscouring elements, the scouring machine should be full to 10-90% byvolume, in particular 40-75% by Volume, With the ceramic articles to besmoothed, plus the scouring elements. The scouring elements : ceramicarticles to be smoothed weight ratio may be 0:1 to 5:1, in particular1:1 to 3:1. A further excess of scouring elements is not harmful, butonly uneconomic. The use of scouring elements is also appropriate if thesliding and sealing face, although easily accessible, is curved. Thisapplies, for example, to cylindrical outer surfaces of plungers andprotective sleeves for shafts. For components having a planar slidingand sealing face, the use of scouring elements is not necessary.

Using the process according to the invention for ceramic objects, it ispossible to obtain surfaces whose mean roughness Ra is between 0.1 and0.8 μm, preferably 0.3-0.8 μm. If the components have a planar slidingand sealing face, an evenness of at least 0.3, preferably at least 0.6μm and a maximum of 1.2 μm, preferably a maximum of 0.8 μm, can beproduced. This applies, in particular, to aluminum oxide objects. Inthis case, the content of Al₂ O₃ is not crucial. For example, groundaluminum oxide components whose Al₂ O₃ content is greater than 80%, evenbetter greater than 90%, in particular greater than 92%, preferablygreater than 94%, can be employed. Components having a content ofgreater than 96%, greater than 98%, greater than 99% and greater than99.5% by weight of Al₂ O₃ can also be used. The higher the Al₂ O₃content, the better the strength values of the component. According toDIN 4762, the mean roughness Ra is known as the arithmetic meanroughness.

In the case of curved surfaces (for example cylinders, spheres andcones), the term evenness is replaced by the term surface accuracy,which indicates the maximum difference between the measured dimensionsand the dimensions given by the mathematically defined shape of thearticle.

In scanning electron microscope photomicrographs, the surfaces producedby the process according to the invention exhibit a surface havingrounded peaks. This surface has a considerably smaller number of sharpedges than a surface of the same mean roughness which has been producedby polishing (for example using diamond polishing paste). This ispossibly due to the substantially lower working pressure during abrasivepolishing.

Valve washers which have been produced by the process according to theinvention require a displacement force which is up to 50% less thanpolished or lapped valve washers of the same composition. They areleakproof to liquid media in the range 0-20 bar. With respect to thesealing behavior against water, no differences can be determinedcompared to polished valve washers, at least up to a pressure of 6 bar.

Ceramic articles can be produced with reduced labor costs by thespecified process. A further advantage is that, in simple apparatuses, alarge number of units can be smoothed per unit time. It is surprisingthat hardly any of the brittle ceramic parts are damaged or destroyedduring abrasive polishing. It is further proposed in German Patent1,949,318, column 9, to lap valve washers made from aluminum oxide inorder to provide them with an extremely smooth surface.

Abrasive-polished surfaces are less shiny than polished surfaces, butmore than ground surfaces. They appear matt and do not reflect.

It has become apparent that it is even possible to obtain meanroughnesses (Ra) of 0.1 to 0.3 μm (and naturally greater) using theprocess according to the invention if the base material comprises atleast 96%, in particular at least 98%, of AL₂ O₃, and the meancrystallite size (in accordance with ASTME 112-74) of the aluminum oxidedoes not exceed a size of 8 microns. Similar values can be obtained forsilicon carbide elements. Pure contents of greater than 99%, preferablygreater than 99.5%, in particular at least 99.7%, by weight of Al₂ O₃are particularly favorable. It is favorable if the ceramic material usedis free of pores. It is furthermore preferred if the mean crystallitesize is in the range 2 to 6, in particular 3 to 5, μm. Surprisingly,aluminum oxide parts having an abrasive-polished sliding and sealingface require considerably lower displacement forces than identical partsof the same mean roughness whose sliding and sealing face has beenproduced by polishing.

The invention furthermore relates to an aluminum oxide sealing componentfor a machine which is free from ZrO₂ and HfO₂, the component being infrictional contact and having at least one planar sliding and sealingface, wherein the sliding and sealing face has a mean roughness (Ra) ofgreater than 0.1 and not more than 0.8 μm, as well as an evenness of0.3-1.2 μm, and is nonreflective. The roughness is preferably 0.3-0.8μm. The evenness of this sliding and sealing face is, in at least onemeasurement direction, in the range from at least 0.3, in particular atleast 0.6, μm to a maximum of 1.2 μm, preferably a maximum of 0.8 μm.For example, steatite or silicon carbide components can be produced.Components based on aluminum oxide are preferred, in particularcontaining at least 80% by weight, preferably at least 90% by weight ofAl₂ O₃. Materials made from Al₂ O₃ containing zirconium oxide and/orhafnium oxide are known from European Patent 043,456.

The shape of components according to the invention is not crucial. Theonly essential factor is the presence of a sliding and sealing face. Itis intended that this machine part will later be in flat contact withanother component. It should be possible for the two components to bemoved mutually and the gap formed by them should be leakproof to fluids,such as, for example, water under pressure.

Preferably, the component according to the invention has the shape of adisk, of a cylinder or of a hollow cylinder.

The invention therefore relates also to a ceramic sealing component fora machine, the ceramic not having a content of ZrO₂ and HfO₂, thecomponent being in frictional contact and having at least one slidingand sealing face, wherein the component has a cylindrical peripheralsurface which is designed as a sliding and sealing face, a roughness(Ra) of greater than 0.1 and not more than 0.8 μm, as well as a surfaceaccuracy of 0.3-1.2 μm, and is nonreflective.

In a refinement of this invention, the components have the shape of acylindrical tube and can be used as piston skirts if the peripheralsurface of the cylindrical tube is designed as a sliding and sealingface. The front faces of the tubes are preferably likewise designed as asliding and sealing face. In a further embodiment, the componentaccording to the invention has the shape of a hollow cylinder which isclosed at one end, whose peripheral surface is designed as a sliding andsealing face. A component of this type can be employed, for example, asa plunger in high-pressure piston pumps. It is preferred if the frontface of the hollow cylinder, in addition, is designed as a sliding andsealing face. In these cylindrical parts, the length : diameter ratio ispreferably at least 1, in particular at least 2.

For valve washers, as installed in mixing valves or mixing faucets inthe sanitary area for regulating liquid flows, a wide variety of ceramicwashers are in use. In this case, at least one side of the washer has asliding and sealing face. In the finished control elements, at least twosuch valve washers are in movable contact with one another, the slidingand sealing faces in each case being in sliding and sealing contact withone another. Diskshaped ceramic components having a sliding and sealingface are preferred, the thickness of the disk in most cases being 1-10mm, in particular 2-5 mm. It is preferred if the F⁰.5 :D ratio in thesedisks is 3-12, in particular 5-9, where F is the surface area of a diskdefined by the external dimensions, and D is the thickness of the disk.Disks having the ratio specified can be processed highly successfully inscouring vibrators. This applies, in particular, to disks of diameters3-50 mm.

A valve washer for sanitary mixing faucets is frequently round orapproximately round. It may have recesses on the periphery. In at leastone washer of a control element, at least one channel for a liquid isprovided. Usually, however, several channels are present for the liquidsto be mixed and the mixed liquid. These channels connect the two sidesof the valve washer. Valve washers having three openings are illustratedin German Patent 1,291,957. It is preferred if the sum of thecross-sections of the individual channels on each side is 5-45% of thewasher surface area. If the proportion of the channel area is greater,the mechanical stability of the valve washers decreases. The counterpartto valve washers having channels are also valve washers having hollowsarranged on the inside of the face.

It is preferred if the two sides of the valve washer are designed as anabrasive-polished sliding and sealing face.

In the case of valve cartridges, which contain two sealing washers infrictional contact with one another and a sealing O-ring on the rear ofthe movable washer, the danger of damage to the O-ring can beconsiderably reduced using washers which are smooth on both sides.

Suitable sliding rings in sliding ring gaskets are ceramic components inthe form of a cylindrical ring washer. These rotationally symmetricalparts exhibit a rectangular outline on projection perpendicular to theaxial direction. At least one, but preferably both ring-shaped sides ofthe washer are designed as a sliding and sealing face. The externaldiameter: washer thickness ratio is in most cases 2.6-15, in particular3-11, preferably 3.5-9. It is also possible to produce angled rings,which can likewise be used as sliding elements in sliding ring gaskets,by the process according to the invention. These rotationally symmetricceramic rings exhibit a T-shaped outline on projection perpendicular tothe axial direction. The ring face having the larger surface area, inparticular, is designed as the sliding face. The external diameter :thickness ratio is about 2-10. Preferred ranges for this ratio are 2.2to 7, 2.8 to 5.6 and 3 to 5.

The invention is illustrated in greater detail by the example.

EXAMPLE

Aluminum oxide powder containing 96% by weight of Al₂ O₃ is ground for48 hours in the presence of water and grinding stones. With addition of2% of a water-soluble binder, such as polyvinyl alcohol ormethylcellulose, a slurry is prepared and sprayed. The granules obtainedare transfer-red into a dye and shaped into a green element in a dryautomatic press at a pressure of 15 kp/mm². Sintering takes place at1700°-1750° C. The washers obtained are ground until level andplane-parallel.

The washers obtained are circular (diameter 17 mm) due to the shape ofthe die. They have 2 small lugs on the outer periphery and on the insidean approximately semi-crescent shaped channel on the inside. The area ofthe channel is about 25% of the entire washer surface area. The washersare 2.3 mm thick. About 40,000 units of such small water sealing washersare transferred into a scouring machine (William Boulton Ltd. Type FM3).The degree of filling is about 15% by volume. The machine issubsequently filled to the top with 70L of water. 1 g. of soft soap areadded per liter of water. The speed of the motor is set to 1440 min⁻¹and the unbalance to 15°. After abrasive polishing for 40 hours, theaqueous phase is removed and the contents are washed and dried. Thesurface of the abrasive-polished water-sealing washer has a roughness of0.5-0.7 μm. The evenness of the sliding and sealing face of the sealingwasher is 1-2 helium light bands (=0.3 to 0.6 μm.

We claim:
 1. A process for the production of ceramic sealing componentsfor a machine, the components being in frictional contact and having atleast one planar or curved sliding and sealing face, comprising thesteps of shaping green ceramic articles, sintering, grinding andsubsequently smoothing the sliding and sealing faces formed, wherein thearticles are smoothed by abrasion with each other in the presence of anaqueous phase, without using scouring elements, until a mean roughnessof greater than 0.1 and not more than 0.8 μm is reached, the articlesused being shaped in such a way that the sliding and sealing faces ofthe articles come in contact with the other articles during smoothing.2. The process as claimed in claim 1, wherein the abrasive polishingtakes place without prior washing of the ground ceramic articles.
 3. Aprocess for the production of a ceramic sealing component for a machine,the component being in frictional contact and having at least one planaror curved sliding and sealing face, comprising the steps of shaping agreen ceramic article, sintering, grinding and subsequently smoothingthe sliding and sealing face formed, wherein the article is smoothed inthe presence of an aqueous phase containing scouring elements, but inthe absence of finely divided abrasively acting substances until a meanroughness of greater than 0.1 and not more than 0.8 μm is reached, andwherein at least one surface of the article to be smoothed is in theinterior of the article.
 4. The process as claimed in claim 1, whereinthe water contains organic additives having a surface activity.
 5. Theprocess as claimed in claim 1, wherein the articles are aluminum oxideceramic articles.
 6. The process as claimed in claim 1, wherein theapparatus used for smoothing is filled to the extent of 10 to 90% byvolume with ceramic articles to be smoothed.
 7. The process as claimedin claim 1, wherein the sliding and sealing faces of sliding rings orregulating washers are smoothed.
 8. An aluminum oxide sealing componentfor a machine which is free from ZrO₂ and HfO₂, the component being infrictional contact and having at least one planar sliding and sealingface, wherein the sliding and sealing face has a mean roughness (Ra) ofgreater than 0.1 and not more than 0.8 μm, as well as an evenness of0.3-1.2 pm, and is non-reflective.
 9. A component as claimed in claim 8,wherein the evenness of the sliding and sealing face is in the range 0.3to 0.8 μm.
 10. A ceramic sealing component for a machine which does notcontain ZrO₂ and HfO₂, the component being in frictional contact andhaving at least one sliding and sealing face, wherein the component hasa cylindrical peripheral surface which is designed as a sliding andsealing face, a roughness (Ra) of greater than 0.1 and not more than 0.8μm, as well as a surface accuracy of 0.3-1.2 μm, and is nonreflective.11. A component as claimed in claim 10, form of a cylindrical tube whoseperipheral surface is designed as a sliding and sealing face.
 12. Acomponent as claimed in claim 10, which has the shape of a hollowcylinder which is closed at one end and whose cylindrical surface isdesigned as a sliding and sealing face.
 13. A component as claimed inclaim 12, wherein the front face of the hollow cylinder, has a meanroughness (Ra) of greater than 0.1 and a maximum of 0.8 μm.
 14. Acomponent as claimed in claim 8, which has a disk shape and wherein atleast one side of the disk has a sliding and sealing face.
 15. Acomponent as claimed in claim 14, wherein the disk has a thickness of 1to 10 mm.
 16. A component as claimed in claim 14, wherein the F⁰.5 :Dratio is 3 to 12, where F is the surface area of the disk, defined bythe external dimensions of the disk, and D is the thickness of the disk.17. A component as claimed in claim 14, wherein the two sides of thedisk are connected by at least one channel.
 18. A component as claimedin claim 17, wherein the sum of the cross-sections of the individualchannels on each side is 5 to 45% of the disk surface area.
 19. Acomponent as claimed in claim 8, which has the shape of a shortcylindrical tube, at least one ring-shaped front face being designed asa sliding and sealing face and the external diameter : tube thicknessratio being at least 2.6.
 20. A component as claimed in claim 10,wherein the cylinder length : diameter ratio is at least
 1. 21. Acomponent as claimed in claim 8, which has an aluminum oxide content ofgreater than 80% by weight.
 22. The process as claimed in claim 3,wherein the water contains organic additives having a surface activity.23. The process as claimed in claim 3, wherein the articles are aluminumoxide ceramic articles.
 24. The process as claimed in claim 3, whereinthe apparatus used for abrasive polishing is filled to the extent of 10to 90% by volume with ceramic articles to be smoothed.
 25. The processas claimed in claim 3, wherein the sliding and sealing faces of slidingrings or regulating washers are smoothed.
 26. A component as claimed inclaim 14, wherein the F⁰.5 :D ratio is 5 to
 9. 27. A process as claimedin claim 3, wherein the scouring elements are selected from the groupconsisting of spheres and rods of ceramic material.
 28. A process asclaimed in claim 27, wherein the scouring elements are selected from thegroup consisting of spheres having a diameter of 3 to 10 mm and rodshaving a length of 3 to 10 mm.
 29. A process as claimed in claim 3,wherein the scouring elements range in size from 3 to 10 mm.
 30. Aprocess as claimed in claim 3, wherein the articles undergo tumblingmovements during the smoothing step.
 31. A process as claimed in claim3, wherein the articles undergo short oscillating movements of highfrequency during the smoothing step.
 32. A process as claimed in claim8, wherein the component has an aluminum oxide content of at least 96%and the aluminum oxide has a mean crystalline size which does not exceed8 microns.
 33. A process as claimed in claim 8, wherein the componenthas an aluminum oxide content of at least 98% and the aluminum oxide hasa mean crystalline size which does not exceed 8 microns.
 34. A processas claimed in claim 32, wherein the means surface roughness is between0.1 to 0.3 microns.
 35. A process as claimed in claim 27, wherein thescouring elements and the article to be smoothed are made of the sameceramic material.